A key tool called "MemCal" is developed to support the simulation of membrane-based gas separation processes using Aspen HYSYS. By integrating "MemCal" with the simulator, users can simulate complex multi-component/multi-stage processes, perform sensitivity studies, and utilize the default HYSYS features to cost and optimize processes. Industrially, "MemCal" can be used to simulate the separation of air, biogas, natural gas, olefins-paraffins and other gases. The CO2/CH4 binary mixture separation was demonstrated in this manuscript as the first and the simplest application of "MemCal". The simulation of various multi-stage processes suggested that the treatment cost can be dropped by optimizing the separation load distribution among the stages. For CO2/CH4 separation, the simulations suggested that the optimized two-stage process plus recycle resulted in the least separation cost. The installation of a third stage for boosting hydrocarbons recovery was found unjustified when membranes equivalent to or better than cellulose acetate are adopted. (C) 2018 Elsevier Ltd. All rights reserved.