This paper investigates the application of two techniques to improve the proportional-integral (PI) control performance when implemented to stabilize the operation of gas-phase polyethylene reactors around an optimal operating point which is open-loop unstable. The first technique deals with improving the performance of a single input-single output control loop through employing an on-line adaptive tuning strategy. The purpose of the tuning strategy is to force the closed-loop response to fit inside a desired time-domain specification envelope by automatically adapting the PI setting values. The second technique deals with improving the multiple input-multiple output control performance by simple selection of the proper control structure design. The objective of the proposed control design is to avoid the procedure of screening various control structure candidates. Moreover, because there are more inputs than outputs, a split-range configuration is used to utilize all available manipulated variables. This configuration allows for tight control. Simulation results demonstrated the success of the proposed methods to provide a better regulatory control performance when compared to those that have been reported in earlier work.