An olefin polymerization process comprising polymerization of at least one olefin monomer in more than one polymerization zone of one or more reactors wherein the polymerization process is carried out in an apparatus comprising: a) one or more reactor vessels to which olefin monomer and catalyst components can be added and which contain an agitated bed of forming polymer particles; b) one or more valve-controlled ports in the reactor vessels through which catalyst components and quench liquid are added, wherein the catalyst components and quench liquid are added directly into and onto the bed of forming polymer particles; c) a recycle gas feed system connected to one or more valve-controlled ports in the reactor vessels through which olefin monomer and quench liquid are provided to the reactor vessels, wherein the recycle gas feed system comprises: 1) a means for removing reactor off-gases along the top of the reactor; 2) a first separator for separating the quench liquid, polymer fines and catalyst components from polymerization monomer and hydrogen; and 3) a second separator for separating the polymerization monomer and hydrogen into separate recycle streams; d) one or more valve-controlled ports located underneath the polymer bed through which the olefin monomer and hydrogen gas are introduced to the reactor vessels; e) a take-off barrier situated at the exit end of the reactor through which solid polymer product is removed; f) a device for removing solid polymer product from the take-off barrier; g) a finishing apparatus connected to the take-off barrier for adding suitable additives to the solid polymer product; and h) an extruder for incorporating the additives into the polymer by heating and mechanical shear and extruding the heated polymer through a die to form discrete pellets; wherein the process comprises the steps of: 1) introducing one or more streams of olefin monomer having hydrogen concentrations of 0 mole% to 70 mole% into a first polymerization zone; 2) introducing one or more streams of olefin monomer having hydrogen concentrations of 0 mole% to 70 mole% into a subsequent polymerization zone; and 3) removing a polymer product having a polydispersity of from 5 to 20 from the last polymerization zone, wherein hydrogen gas is provided to the reactor vessels by the recycle gas feed system in an amount that is adjusted to maintain a constant melt flow rate for the polymer product and the required hydrogen distribution is controlled by varying the number of hydrogen-lean and hydrogen-rich recycle liquid monomer injection points as well as the concentration of hydrogen in the hydrogen-lean streams. a) one or more reactor vessels to which olefin monomer and catalyst components can be added and which contain an agitated bed of forming polymer particles; b) one or more valve-controlled ports in the reactor vessels through which catalyst components and quench liquid are added, wherein the catalyst components and quench liquid are added directly into and onto the bed of forming polymer particles; c) a recycle gas feed system connected to one or more valve-controlled ports in the reactor vessels through which olefin monomer and quench liquid are provided to the reactor vessels, wherein the recycle gas feed system comprises: 1) a means for removing reactor off-gases along the top of the reactor; 2) a first separator for separating the quench liquid, polymer fines and catalyst components from polymerization monomer and hydrogen; and 3) a second separator for separating the polymerization monomer and hydrogen into separate recycle streams; 1) a means for removing reactor off-gases along the top of the reactor; 2) a first separator for separating the quench liquid, polymer fines and catalyst components from polymerization monomer and hydrogen; and 3) a second separator for separating the polymerization monomer and hydrogen into separate recycle streams; d) one or more valve-controlled ports located underneath the polymer bed through which the olefin monomer and hydrogen gas are introduced to the reactor vessels; e) a take-off barrier situated at the exit end of the reactor through which solid polymer product is removed; f) a device for removing solid polymer product from the take-off barrier; g) a finishing apparatus connected to the take-off barrier for adding suitable additives to the solid polymer product; and h) an extruder for incorporating the additives into the polymer by heating and mechanical shear and extruding the heated polymer through a die to form discrete pellets; 1) introducing one or more streams of olefin monomer having hydrogen concentrations of 0 mole% to 70 mole% into a first polymerization zone; 2) introducing one or more streams of olefin monomer having hydrogen concentrations of 0 mole% to 70 mole% into a subsequent polymerization zone; and 3) removing a polymer product having a polydispersity of from 5 to 20 from the last polymerization zone,