The optimal design of a membrane gas separation process requires minimizing several objective functions subject to nonlinear relationships among the optimizing variables. This article describes a novel software product, named Optimal Membrane-Process Design (OMPD), for the optimal design of membrane gas separation processes. The product generates several potential process design configurations and then searches the process design parameters and operating conditions spaces to arrive at optimal design specifications and operating conditions. It is able to consider every type and any number of operational, compositional, and economical objective functions in a computationally cost-effective manner. It calculates all Pareto optimal solutions in a single trial. It can optimize any number of membrane units arranged in multi-step and/or multi-stage configurations. It optimally places pairs of adjacent membrane units, either two-step or two-stage, while simultaneously considering several membrane types.