Polymerisation catalyst

The present invention is a process and an apparatus for the gas phase polymerization of olefin(s) using a fluidised-bed reactor ( 1 ) from which, at the top part, is withdrawn a recycling gas stream which comprises unreacted gas and entrained solid particles and which is moved by virtue of a compressor ( 4 ), which is cooled by passing into a heat exchanger ( 7 ) and which is reintroduced into the reactor at the bottom part ( 1 a ) of the latter. The heat exchanger ( 7 ) is a multitubular exchanger successively comprising an inlet chamber ( 6 ), a bundle of tubes ( 9 ) with a horizontal longitudinal axis and an outlet chamber ( 10 ) equipped with a discharge orifice ( 11 ). In the outlet chamber ( 10 ) the flow of the gas stream is accelerated up to the orifice ( 11 ) and is directed at any point in the said chamber along a horizontal axis and/or towards the bottom part ( 1 a ) of the said chamber, so as to create, in the bottom part of the said chamber, a sweeping stream which flows horizontally or with a downward inclination and without obstacle up to the orifice and which promotes the entrainment of the particles up to the orifice.