In an industrial gas-phase polymerization process, dynamics change globally due to grade changes and load changes, and these changes present a difficult control problem. In this paper, a nonlinear physical model was effectively used for the control system design. First, we developed a set of physical models of an industrial ethylene polymerization process with reference to McAuley's model. Parameters in the model were adjusted to simulate the actual process behavior. Second, in the control system design, the system is regarded as a two-input and two-output system. The two inputs are the feed rates of fresh hydrogen and butene, and the two outputs are cumulative melt index and density. An optimal servo controller with integral actions is designed according to the optimal regulator theory using a model linearized at a nominal operating point. Third, we examined the changes of process dynamics under typical operating points for different grade products. As a result, a nonlinear compensator was attached to the optimal servo controller to cope with remarkable changes in the process gain due to the product grade. This method resulted in good control performances during various grade changes.