This paper addresses the polyethylene quality transition in gas phase fluidized bed reactors using feedback control. Specifically, a nonlinear model predictive controller is utilized to steer the polymer density and the entire molecular weight distribution to follow predefined targets. The hydrogen and co-monomer inflows are used as the manipulated variables. In this work, the molecular weight distribution is modeled as a function of the reaction kinetics and hydrogen to monomer ratio. The simulation results indicated successful implementation of the control algorithm to attain the desired molecular weight distribution and density, even with the existence of modeling errors. Economically undesirable operations such as excessive purge and/or reduced production rate are avoided by the limited use of monomer and catalyst inflows.