A gas phase polymerization assembly comprising a gas phase polymerization reactor having at least one inlet and at least one outlet, a circulation gas compression unit having an inlet and an outlet, whereby the inlet of the circulation gas compression unit is fluidly connected to the at least one outlet of the gas phase polymerization reactor by an unreacted gas line; a flush gas compression unit having an inlet and an outlet, wherein the inlet of the flush gas compression unit is fluidly connected to the outlet of the circulation gas compression unit by a pressurized unreacted gas line; a circulation line fluidly connecting the pressurized unreacted gas line with the at least one inlet of the gas phase polymerization reactor; a flush gas withdrawal line connected to the outlet of the flush gas compression unit.