TITLE: “FLUIDIZED-BED REACTOR FOR THE GAS-PHASE POLYMERIZATION OF OLEFINS” D E S C R I P T I O N TECHNICAL FIELD The present invention relates to a fluidized-bed reactor for the gas-phase polymerization olefins and a process of preparing an olefin polymer. BACKGROUND OF THE INVENTION Gas-phase polymerization processes are economical processes for the preparation of polyolefins such as homopolymers of ethylene or propylene or copolymers of ethylene and/or propylene with other olefins. Fluidized-bed reactors for carrying out such processes have been known for a long time. These reactors contain a bed of polymer particles which is maintained in a fluidized state by an upward flow of a fluidizing gas. Customary reactors comprise, inter alia, a reactor space in the form of an inner chamber of a vertical cylinder provided. These reactors have a recycle gas line, in which coolers for removing the heat of polymerization, a recycle gas compressor and, if desired, further elements such as a cyclone for removing fine polymer dust are installed. Monomers consumed by the polymerization reaction are normally replaced by adding make-up gas to the recycle gas stream. To achieve a homogeneous distribution of the fluidizing gas in the bed of growing polymer particles, some reactors are equipped with a gas distribution grid, sometimes also called gas fluidization grid or distribution plate. Such a gas distribution grid is a device provided with openings which dispense into the bed a gas stream introduced under the grid itself. The grid also acts as support for the bed when the supply of gas is cut off. The gas distribution grid can be configured as a perforated or porous plate, sometimes in combination with an upstream flow divider. It is possible to arrange roof-shaped deflector plates above the openings in the distributor plate, as for example disclosed in EP 0697421 A1, or to cover the openings with a cap as described in EP 0600414 A1. The geometry of the gas distribution grid may also deviate from a plate. EP 0088638 A2 discloses a gas distributor for a fluidized- bed reactor which has a double cone-body. WO 2008/074632 A1 describes a gas distribution grid which has the form of an inverted cone. Due to the high amount of circulated fluidization gas and the consequently large size of the gas inlet nozzle, a relatively large volume is required below the gas distribution grid. 1 FE7443 WO 01 Over time, the fines present in the fluidizing gas can accumulate in the space under the reactor grid. These reactive fines in stagnant conditions can then develop polymer agglomerates which can eventually become too bulky and thus jeopardize the operability and reliability of the reactor system. There is accordingly a need to provide a fluidized-bed reactor which allows fine polymer particles carried over by the fluidizing gas to be easily transported back into the fluidized bed of polymer particles. The object of the present invention is to provide a fluidized-bed reactor and a process for preparing an olefin polymer that allow the drawbacks of the known art to be at least partially overcome, and which are, at the same time, simple and inexpensive to implement. SUMMARY According to the invention there is provided a fluidized-bed reactor and a process of preparing an olefin polymer according to the appended independent claims and, preferably, according to any one of the claims directly or indirectly depending on the independent claims. BRIEF DESCRIPTION OF THE FIGURES The invention is hereinafter described with reference to the accompanying drawings, which depict some non-limiting embodiments thereof, wherein: - figure 1 is schematic and side view of a fluidized-bed reactor in accordance with the invention; - figure 2 is a lateral cross-section of a part of the fluidized-bed reactor of figure 1; - figure 3 is a plan view of the part of figure 2 with some details removed for clarity; - figure 4 is a front cross-section of a detail of figure 3; - figure 5 is a lateral cross-section of the detail of figure 4; - figure 6 is a lateral cross-section of a different embodiment of the part of figure 2; and - figure 7 is a lateral cross-section of a further different embodiment of the part of figure 2. DETAILED DESCRIPTION In figure 1, the numeral 1 indicates as a whole a fluidized-bed reactor for the gas- phase polymerization of olefins. The fluidized-bed reactor 1 has an inner chamber 2, which, in turn, has at least one lower portion 3 and at least one upper portion 2 FE7443 WO 01 4. The fluidized-bed reactor 1 comprises: at least one lateral wall 5, which has an inner surface 6 delimiting the inner chamber 2 laterally (at least partially); a gas distribution grid 7, which is located inside the inner chamber and which (at least partially) separates the lower portion 3 from the upper portion 4; and a gas recycle line 8, which has a first end 9 connected to the inner chamber 2 at the upper portion 4 and a second end 10 connected to the inner chamber 2 at the lower portion 3. More precisely but not necessarily, the gas recycle line 8 is configured to convey the recycled part of the fluidizing gas from the upper portion 4 of the inner chamber 2 through the upper wall 15 and the fluidizing gas through the lateral wall 5 to the lower portion 4. In particular, olefins which may be polymerized in the fluidized-bed reactor 1 of the present disclosure are especially 1-olefins, i.e. hydrocarbons having terminal double bonds, without being restricted thereto. Preference is given to nonpolar olefinic compounds. Particularly preferred 1-olefins are linear or branched C ₂ -C ₁₂ -1-alkenes, in particular linear C ₂ -C ₁₀ -1-alkenes such as ethylene, propylene, 1- butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-decene or branched C ₂ - C ₁₀ -1-alkenes such as 4-methyl-1-pentene, conjugated and nonconjugated dienes such as 1 ,3-butadiene, 1 ,4-hexadiene or 1 ,7-octadiene. It is also possible to polymerize mixtures of various 1-olefins. Suitable olefins also include those in which the double bond is part of a cyclic structure which can have one or more ring systems. Examples are cyclopentene, norbornene, tetracyclododecene or methylnorbornene or dienes such as 5-ethylidene-2- norbornene, norbornadiene or ethylnorborna- diene. It is also possible to polymerize mixtures of two or more olefins. The fluidized-bed reactor 1 is particularly suitable for the homopolymerization or copolymerization of ethylene or propylene and is especially preferred for the homopolymerization or co-polymerization of ethylene. Preferred comonomers in propylene polymerization are up to 40 wt.% of ethylene, 1-butene and/or 1- hexene, preferably from 0.5 wt.% to 35 wt.% of ethylene, 1-butene and/or 1- hexene. As comonomers in ethylene polymerization, preference is given to using up to 20 wt.%, more preferably from 0.01 wt.% to 15 wt.% and especially from 0.05 wt.% to 12 wt.% of C ₃ -C ₈ -1-alkenes, in particular 1-butene, 1-pentene, 1- hexene and/or 1-octene. Particular preference is given to polymerizations in which ethylene is copolymerized with from 0.1 wt.% to 12 wt.% of 1-hexene and/or 1-butene. 3 FE7443 WO 01 In an advantageous but non-limiting embodiment of the present disclosure, the polymerization is carried out in the presence of an inert gas such as nitrogen or an alkane having from 1 to 10 carbon atoms such as methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane or n-hexane or mixtures thereof. The use of nitrogen or propane as inert gas, if appropriate in combination with further alkanes, is preferred. In especially advantageous embodiments of the present disclosure, the polymerization is carried out in the presence of a C ₃ -C ₅ alkane as polymerization diluent and most preferably in the presence of propane, especially in the case of homopolymerization or copolymerization of ethylene. The reaction gas mixtures within the reactor additionally comprise the olefins to be polymerized, i.e. a main monomer and one or more optional comonomers. In an advantageous embodiment of the present disclosure, the reaction gas mixture has a content of inert components from 30 to 99 vol.%, more preferably from 40 to 95 vol.%, and especially from 45 to 85 vol.%. In another advantageous embodiment of the present disclosure, especially if the main monomer is propylene, no or only minor amounts of inert diluent are added. The reaction gas mixture may further comprise additional components such as antistatic agents or molecular weight regulators like hydrogen. The components of the reaction gas mixture may be fed into the gas-phase polymerization reactor or into the recycle gas line in gaseous form or as liquid which then vaporizes within the reactor or the recycle gas line. According to some non-limiting embodiments, the polymerization of olefins is carried out using all customary olefin polymerization catalysts. That means that, for example, the polymerization can be carried out using Phillips catalysts based on chromium oxide, using Ziegler or Ziegler-Natta-catalysts, or using single-site catalysts. For the purposes of the present disclosure, single-site catalysts are catalysts based on chemically uniform transition metal coordination compounds. Furthermore, it is also possible to use mixtures of two or more of these catalysts for the polymerization of olefins. Such mixed catalysts are often designated as hybrid catalysts. The preparation and use of these catalysts for olefin polymerization are generally known. Advantageous examples of catalysts of the Ziegler type comprise a compound of titanium or vanadium, a compound of magnesium and optionally an electron donor compound and/or a particulate inorganic oxide as a support material. Catalysts of the Ziegler type are usually used in the presence of a cocatalyst. Examples of cocatalysts are organometallic compounds of metals of Groups 1 , 2, 12, 13 or 14 of the Periodic Table of Elements, in particular organometallic 4 FE7443 WO 01 compounds of metals of Group 13 and especially organoaluminum compounds. Preferred cocatalysts are for example organometallic alkyls, organometallic alkoxides, or organometallic halides. Advantageous examples of organometallic compounds comprise lithium alkyls, magnesium or zinc alkyls, magnesium alkyl halides, aluminum alkyls, silicon alkyls, silicon alkoxides and silicon alkyl halides. More advantageously, the organometallic compounds comprise aluminum alkyls and magnesium alkyls. Even more advantageously, the organometallic compounds comprise aluminum alkyls, most advantageously trialkylaluminum compounds or compounds of this type in which an alkyl group is replaced by a halogen atom, for example by chlorine or bromine. Examples of such aluminum alkyls are trimethylaluminum, triethylaluminum, tri-isobutylaluminum, tri-n-hexylaluminum or diethylaluminum chloride or mixtures thereof. According to some non-limiting embodiments, the fluidized-bed reactor of the present disclosure is operated at pressures of from 0.5 MPa to 10 MPa, advantageously from 1.0 MPa to 8 MPa and in particular from 1.5 M Pa to 4 M Pa. The polymerization is advantageously but not necessarily carried out at temperatures of from 30 °C to 60 °C, particularly advantageously from 65 °C to 125 °C, with temperatures in the upper part of this range being preferred for preparing ethylene copolymers of relatively high density and temperatures in the lower part of this range being preferred for preparing ethylene copolymers of lower density. According to some non-limiting embodiments, the polymerization in the fluidized- bed reactor is also carried out in a condensing or super-condensing mode, in which part of the circulating reaction gas mixture is cooled to below the dew point and returned to the reactor either separately as a liquid and a gas-phase or together as a two-phase mixture in order to make additional use of the enthalpy of vaporization for cooling the reaction gas. The gas recycle line 8 is configured to feed a fluidizing gas, which comprises fresh olefins monomers (added along the gas recycle line 8) and a recycled part (taken from the inner chamber 2) containing recycled unreacted and/or partially reacted olefins, to the lower portion 3 of the inner chamber 2. With particular reference to figure 2, the gas distribution grid 7 is (in particular, comprises a plurality of openings 11, that are) configured to allow the passage of 5 FE7443 WO 01 the fluidizing gas from the lower portion 3 to the upper portion 4 of the inner chamber 2. The fluidized-bed reactor 1 also comprises a polymer discharge channel 17, which is configured to discharge a polymer, obtained inside the upper portion 4, from the upper portion 4 itself (in particular, downward) in a discharge direction D (in particular, through the lower portion 3). More precisely but not necessarily, the discharge direction D is substantially vertical (and, in particular, parallel to the longitudinal extension of the fluidized-bed reactor). The fluidized-bed reactor 1 further comprises a divider wall DW located around at least a stretch of the polymer discharge channel 17 so as to delimit a first area A of the inner chamber 2 and prevent said the olefins monomers and the recycled unreacted and/or partially reacted olefins entering the lower portion 3 through the second end 10 to reach the first area A. The divider wall DW has at least a first part DW’ which is crosswise (in particular, perpendicular) to said discharge direction D. The second end 10 is configured so as to feed the fluidizing gas to the lower portion 3 in a direction crosswise to the discharge direction D. In other words, the second end 10 is configured so that the fluidizing gas exiting the second end 10 moves in a direction crosswise to the discharge direction D. In particular, the second end 10 (of the recycle line 8) is configured so that the recycle line 8 feeds the fluidizing gas exiting the second end 10 itself to the lower portion 3 in a direction crosswise (in particular, substantially perpendicular) to the discharge direction D. In this way, it has been experimentally observed that, surprisingly, there is a reduced development of polymer agglomerates inside the lower portion 3 (in particular, around the discharge channel 17). Please note that it has been experimentally seen that also in the area of the divider wall DW there is a reduced development of polymer agglomerates. It has been supposed that these effects are obtained because the shape of the divider wall DW is such that there are less (narrow) spaces where turbulent eddies and/or a stagnation can take place so that fine particles can accumulate. The direction of the fluidizing gas exiting the second end 10 also has shown to play a role in this context as, supposedly, in this way the movement of the fluid is particularly adapted to the shape of the surfaces (in particular, of the divider wall DW). 6 FE7443 WO 01 In some specific and non-limiting cases, the recycle line 8 comprises a pipe, whose stretch at the second end 10 extends crosswise (in particular, at an angle between 20° and 160°; more in particular, between 60° and 120°; even more in particular, substantially perpendicularly) with respect to the discharge direction D (in particular, to the polymer discharge channel 17). In particular, such a stretch of the pipe of the recycle line 8 is substantially horizontal. According to some embodiments, which are not depicted, the second end 10 is configured so that the fluidizing gas is fed to the lower portion 4 through the lateral wall 5. In particular, the divider wall DW is located (at least partially) at the lower portion 3. According to some non-limiting embodiments, the divider wall DW is a non- pressure-resistant divider. This implies that the volume above the divider wall DW and the volume below the divider wall DW are kept at the same pressure, preferably by a pressure equalization line, and that the divider wall DW does not have to withstand the polymerization pressure within the fluidized-bed reactor. In this respect, in some specific cases, a monomer and/or clean gas (i.e. without polymer fines) is fed to the first area A so that there is no pressure differential between the sides of the divider wall (between the first area A and a second area B – described below in more details). According to some non-limiting embodiments, the openings 11 comprise openings 11’ located at less than 40 mm (in particular, at less than 20 mm; more in particular, less than 5 mm) from the inner surface 6 of the lateral wall 5. Advantageously but not necessarily, the fluidized-bed reactor 1 comprises a lateral support 12, which extends in a loop along the inner surface 6 in contact with the inner surface 6 (and – at least partially – supports the gas distribution grid 7). The gas distribution grid 7 has a peripheral edge 13, which is positioned (at least) partially on (in particular, rests on) and in contact with the lateral support 12. The lateral support 12 has apertures 14, each of which is positioned at under a corresponding opening 11’ and is configured to allow the passage of the fluidizing gas from the lower portion 3 to the upper portion 4 of the inner chamber 2 through the corresponding opening 11’. This ensures, at the same time, sufficient mechanical stability for the grid 7 without, at the same time, hindering the passage through the openings 11’. 7 FE7443 WO 01 Please note that in figure 3, only a part (about one half) of the grid 7 is depicted so as to better show the structure of the lateral support 12. According to some non-limiting embodiments, the openings 11 are formed in such a way that the flow of the fluidizing gas after having passed the openings 11 is substantially parallel to a plane of the gas distribution grid 7 (in particular, substantially tangential to the gas distribution grid 7; more in particular, substantially horizontal). Advantageously but not necessarily, the openings 11 are slots. According to some non-limiting embodiments, the width of the slots (openings 11) is more than their height (in particular, more than the double of their height). More precisely but not in a limiting way, the openings 11 are made as disclosed in patent application WO2008074632 of the same applicant. Advantageously but not necessarily, the divider wall DW has at least one second part DW”, which is connected to the first part DW’ with an obtuse angle E facing the lower portion 3 outside the first area A. Also in this way, a further reduction development of polymer agglomerates inside the lower portion 3 has been surprisingly observed. In some embodiments, which are not limiting, the fluidized-bed reactor 1 comprises an upper wall 15, which delimits a top of the inner chamber 2 and is connected to the lateral wall 5; and a lower wall 16, which delimits a bottom of the inner chamber 2 and is connected to the lateral wall 5. The lower wall 16 is also connected to the divider wall DW (in particular, to the first part DW’ of the divider wall DW) with an obtuse angle D facing the lower portion 3 outside the first area A. In particular, also the second part DW” of the divider wall DW is connected to the lower wall 16. It has been experimentally seen that, surprisingly, implementing these solutions, there is a further reduction of the development of polymer agglomerates inside the lower portion 3 (in particular, at the conjunction between the lower wall 16 and the divider wall DW). 8 FE7443 WO 01 In particular, the first area A is at least partially delimited by the lower wall 16 and the divider wall DW. Advantageously but not necessarily, the first part DW’ is substantially horizontal. According to some non-limiting embodiments, the obtuse angle D is at least 100°, in particular, at least 120° (in particular, up to 170°; more in particular, up to 150°). In addition or alternatively, the obtuse angle E is at least 100° (and, in particular, lower than 150°). With particular reference to figure 6, according to some non-limiting embodiments, the divider wall DW is curved. In such a case, it can be said that the divider wall DW has infinite parts DW’, DW’’…DW ⁿ placed one after the others along the curve. On the contrary, figure 7 shows a non-limiting embodiment, wherein the divider wall DW has just one part DW’. In the embodiment of figure 2, the divider wall DW has the two parts DW’ and DW”. According to some non-limiting embodiments, the fluidized-bed reactor 1 comprises a polymer discharge pipe 17’, which laterally delimits the polymer discharge channel 17 and extends (in particular, from the upper portion 4) through the lower wall 16. In particular, the divider wall DW is crosswise to the longitudinal extension of (an external surface of) the polymer discharge pipe 17’. Additionally or alternatively, the divider wall DW (more precisely but not necessarily, the first part DW’ of the divider wall DW) is connected to an external surface of the polymer discharge pipe 17’ so that said first area A is delimited by the polymer discharge pipe 17’, the divider wall DW and the lower wall 16. According to some non-limiting embodiments, the lateral wall 5 extends substantially straight in the direction of the axis of the fluidized-bed reactor 1 (in particular, of the inner chamber 2). More precisely but not necessarily, the lateral wall 5 extends substantially straight in the discharge direction D. Additionally or alternatively, the lower wall 16 extends substantially crosswise in the direction of the axis of the fluidized-bed reactor 1 (in particular, of the inner 9 FE7443 WO 01 chamber 2). More precisely but not necessarily, the lower wall 16 extends substantially crosswise to the discharge direction D. In some specific and non-limiting cases, the lower wall 16 is rounded. In particular, the lower wall 16 has a portion (in the area of the discharge pipe 17’) substantially perpendicular to the direction D and a portion (connected to the lateral wall 5) with a very small angle (less than 1°) with respect to the direction D. According to some non-limiting embodiments, the lower portion 3 comprises (in particular, consists of) the first area A and a second area B, at (to) which the recycle line 8 is configured to feed a fluidizing gas. The divider wall DW separates the first area A from the second area B (and vice versa). In particular, the second area B is delimited (at least partially) by the divider wall DW, the lower wall 16 and the grid 7 (and possibly the lateral wall 5) and is designed to receive the fluidizing gas from the recycle line 8. In particular, the obtuse angles D and E are inside the second area B (they face the second area B). In some non-limiting cases, the divider wall DW is devoid of acute angles on its surface(s) facing the lower portion 3 outside said first area A. In particular, the divider wall DW is connected to the external surface of the polymer discharge pipe 17’ with an acute angle J facing said first area A of at least 10° (in particular, at least 20°; in particular, up to 50°; more in particular, up to 40°). According to some non-limiting embodiments, the polymer discharge pipe 17’ is substantially parallel to the lateral wall 5. Additionally or alternatively, the polymer discharge pipe 17’ is substantially parallel to the discharge direction D. Advantageously but not necessarily, the polymer discharge pipe 17’ comprises an upper opening 18 integrated into the gas distribution grid 7. In particular, the upper opening 18 of the polymer discharge pipe 17’ is arranged in the center of the gas distribution grid 7. More in particular, the polymer discharge pipe 17’ is configured to discharge the polymer produced inside the upper portion 4. 10 FE7443 WO 01 According to some non-limiting (and not depicted) embodiments, the gas recycle line 8 is configured to convey the recycled part of the fluidizing gas from the upper portion 4 of the inner chamber 2 through the upper wall 15 and the fluidizing gas through the lateral wall 5 to the lower portion 4. According to some non-limiting embodiments, the discharge pipe 17’ comprises regulation means 21, such as a discharge valve, configured to adjust the mass flow rate of polymer discharged from the reactor 1. The opening of the regulation means 21 are continuously adjusted, so as to keep constant the height of the fluidized polymer bed inside the reactor 1. The discharge pipe 17 may be made of a uniform diameter, but advantageously comprises more sections having decreasing diameters in the downward direction. The regulation means 21 are advantageously placed at a restriction between a section of higher diameter and a section of lower diameter as shown in Fig.1. As an alternative the discharge system is as disclosed in the patent application WO2007071527A1 of the same applicant. In specific and non-limiting cases, the gas recycle line 8 is provided with (a compressor 19 and) a heat exchanger 20, which is configured to reduce the heat of the recycled part. Advantageously but not necessarily, the recycle line 8 is provided with a topping up line 22 for feeding the fresh olefins monomers, molecular weight regulators and, optionally inert gases (in particular, to a main pipe 23 of the recycle line 8). More precisely but not necessarily, the topping up line 22 is configured to feed the fresh olefins monomers, molecular weight regulators and, optionally, inert gases (and antistatic agents, mileage improvers, etc.) upstream from the compressor 19 (in particular, between the upper portion 4 and the compressor 19; more in particular, upstream from the heat exchanger 20). Advantageously but not necessarily, the second end 10 of the recycle line 8 and the divider wall DW are configured so that (the flow of) the fluidizing gas exiting the second end 10 (and, in particular entering the lower portion 3; more in particular, entering the second area B) is substantially tangential to (at least a part of) the divider wall DW (in particular, to the first part DW’ and/or the second part DW”). 11 FE7443 WO 01 It has been experimentally observed that, surprisingly, in this way, a production of agglomerates in the divider wall DW is reduced even more consistently. It has been supposed that these effects are obtained because the movement of the fluidizing gas kind of sweeps the surface of the divider wall DW continuously. According to some embodiments, which are not limiting, the gas distribution grid 7 has substantially the form of a lateral surface of a truncated (and inverted) cone. In accordance with a further aspect of the present invention it is also hereby provided a process for preparing an olefin polymer comprising homopolymerizing an olefin or copolymerizing an olefin and one or more other olefins (in particular, at temperatures of from 20 to 200 °C; in particular, at a pressures of from 0.5 to 10 MPa) in the presence of a polymerization catalyst, wherein the polymerization is carried out in the fluidized-bed reactor 1 as above disclosed. According to some non-limiting embodiments, the fluidized-bed reactor 1 comprises the polymer discharge pipe 17’, through which the polymer is continuously discharged. According to some non-limiting embodiments, the polymerization conditions are those conventionally adopted in gas-phase reactors for the olefin polymerization, that is to say a temperature ranging from 60 to 120°C and a pressure ranging from 5 to 40 bar. The gas-phase polymerization process can be combined with conventional technologies operated in slurry, in bulk, or in a gas-phase, to carry out a sequential multistage polymerization process. Therefore, upstream or downstream the polymerization apparatus of the invention, one or more polymerization stages operating in a loop reactor, or in a conventional fluidized bed reactor, or in a stirred bed reactor, can be provided. In particular, gas-phase polymerization reactors having interconnected polymerization zones as described in EP 782 587 and EP 1012195 can be advantageously arranged upstream or downstream the apparatus of the present invention. The gas-phase polymerization process allows the preparation of a large number of olefin powders having an optimal particle size distribution with a low content of fines. The [alpha]-olefins advantageously polymerized by the process of the disclosure have formula CH ₂ =CHR, where R is hydrogen or a hydrocarbon radical having 1-12 carbon atoms. Examples of polymers that can be obtained are: 12 FE7443 WO 01 high-density poly ethylenes (HDPEs having relative densities higher than 0.940) including ethylene homopolymers and ethylene copolymers with [alpha]-olefins having 3 to 12 carbon atoms; linear polyethylenes of low density (LLDPEs having relative densities lower than 0.940) and of very low density and ultra low density (VLDPEs and ULDPEs having relative densities lower than 0.920 down to 0.880) consisting of ethylene copolymers with one or more [alpha]-olefins having 3 to 12 carbon atoms; elastomeric terpolymers of ethylene and propylene with minor proportions of diene or elastomeric copolymers of ethylene and propylene with a content of units derived from ethylene of between about 30 and 70% by weight; isotactic polypropylene and crystalline copolymers of propylene and ethylene and/or other [alpha]-olefins having a content of units derived from propylene of more than 85% by weight; isotactic copolymers of propylene and [alpha]-olefins, such as 1-butene, with an [alpha]-olefin content of up to 30% by weight; impact-resistant propylene polymers obtained by sequential polymerisation of propylene and mixtures of propylene with ethylene containing up to 30% by weight of ethylene; atactic polypropylene and amorphous copolymers of propylene and ethylene and/or other [alpha]-olefins containing more than 70% by weight of units derived from propylene. The gas-phase polymerization process herewith disclosed is not restricted to the use of any particular family of polymerization catalysts. The process can be implemented in any exothermic polymerization reaction employing any catalyst, whether it is supported or unsupported, and regardless of whether it is in pre- polymerized form. The polymerization reaction can be carried out in the presence of highly active catalytic systems, such as Ziegler-Natta catalysts, single site catalysts, chromium-based catalysts, vanadium-based catalysts. Unless expressly indicated to the contrary, the content of the references (articles, books, patent applications etc.) cited in this text is recalled in full herein. In particular, the above-mentioned references are incorporated herein by reference. 13 FE7443 WO 01