The invention relates to a reactor system and process for the catalytic polymerization of olefin monomer and optionally comonomer(s), having one or more inlets for olefin monomer, catalyst, optionally for comonomer, chain growth controllers or chain transfer agents, and/or inert gas, an outlet for gas and an outlet for polymerized particles. The reactor system has at least one fluidized bed unit and at least one moving bed unit, wherein the fluidized bed unit has means for maintaining a fluidized bed in the fluidized bed unit and wherein the moving bed unit is provided with an inlet and an outlet which are connected to the fluidized bed unit, wherein the outlet of the moving bed unit is provided with means for displacing metered quantities of polymerized particles from the moving bed unit into the fluidized bed unit, and to the use of such reactor system.