Multiobjective Pareto optimal solutions have been generated for three grades of nylon 6 being produced in an industrial semibatch reactor. The optimal operating conditions (called preferred solutions) for these three batches are easy to implement and lead to substantial improvements over current practice. The technique used is quite general and can easily be applied to improve the operation of other industrial polymerization reactors or design better (new) reactors. Good mathematical models, which account for the important physicochemical aspects actually operative in a reactor and which have been tested on industrial data, are a prerequisite for such optimization studies.