Membrane process, a relatively new technology among other available techniques, can be used for the purpose of CO 2 capture from natural gas. Over the decades, membrane performance has been described by different mathematical models, but there is limited work done in the field of process simulation where membrane models can be incorporated with other unit operations using commercially available simulator. In this paper, a two dimensional cross flow mathematical model for membrane separation has been incorporated with Aspen HYSYS as a user defined unit operation in order to optimize and design the membrane system for CO 2 capture from natural gas. Parameter sensitivities, along with process economics, have been studied for different design configurations (including recycle streams and multiple stages). It has been observed that double stage with permeate recycle system gives the optimum design configuration due to minimum process gas cost involved with it. © 2011 Elsevier Ltd.