A continuous gas fluidized bed process for the polymerization of olefin monomer(s) wherein the recycled gaseous stream withdrawn from the reactor is divided into two streams (A and B) and that (a) a first stream (A) having been cooled to a temperature at which liquid condenses out is then reintroduced directly into the fluidized bed in the reactor in such a way that, at any time, the condensed liquid is continuously introduced into the bed at a minimum rate of 10 liters of liquid per cubic meter of fluidized bed material per hour, and (b) a second stream (B), which by-passes the above cooling/condensing step, is passed through an exchanger and is then reintroduced into the reactor. The continuous introduction of a liquid into the reactor reduces or even eliminates fouling problems which can be encountered in conventional gas phase olefin polymerization processes.