process for the gas phase polymerization of olefins , in particular , with recycle of fines to the reaction zone . the process includes polymerizing an olefin in a reaction zone in which a bed of polymer particles is maintained in an agitated state by passing a fluid stream through the bed , withdrawing the fluid stream from the reactor after it has passed through the bed of polymer particles , separating entrained solids from the withdrawn fluid stream , and returning the separated entrained solids to the reaction zone . the polymer production rate of the process is at least 40 tonnes / hour , and the separated solids from step are returned to the lower half of the bed of polymer particles in the reaction zone .