The process and the system are used for carrying out the quench-cooled, vapor-phase polymerization of olefin monomer in a horizontally disposed reactor vessel which comprises contacting olefin monomer, or a mixture of olefin monomers, with a polymerization catalyst system in the presence of hydrogen in the reactor vessel to form a polymer product. The improvement comprises introducing a titanium-containing catalyst component of the catalyst system into the top side of the reactor vessel at a point adjacent to the upstream end of the reactor vessel and introducing a cocatalyst plus modifer component of the catalyst system into the top side of the reactor vessel at a distance, downstream of the point of introduction of the titanium-containing catalyst component, which is equal to 25% of the inside diameter of the reactor vessel.