Abstract: The present invention provides a continuous gas fluidised bed process for the polymerisation of olefin monomer selected from (a) ethylene, (b) propylene, (c) mixtures of ethylene and propylene, and (d) one or more other alpha-olefins mixed with (a), (b) or (c), to produce a polymer in a fluidised bed reactor which process comprises:(a) continuously withdrawing from the reactor a gaseous stream comprising unreacted olefin monomer and having entrained therein catalyst and/or polymer particles;(b) continuously recycling said gaseous stream comprising unreacted olefin monomer through a fluidised bed in said reactor in the presence of a polymerisation catalyst under reactive conditions;(c) cooling by means of heat exchanger(s) at least part of said gaseous stream withdrawn from said reactor to a temperature at which liquid condenses out;(d) separating at least part of the condensed liquid from the gaseous stream; and(e) introducing at least part of the separated liquid directly into the fluidised bed, characterise