the present invention provides a polymerization process for producing polymers in a continuous gas phase fluidized bed reactor , wherein a recycle stream is allowed to cool down to below its dew point and the resultant two - phase mixture of the gas and liquid phases is recycled to the reactor , the inert condensable agent introduced into the recycle stream to raise the dew point being saturated hydrocarbons , characterized in that the saturated hydrocarbon is cyclopentane .