Before natural gas is injected to the natural gas grid, contaminants, such as nitrogen and CO2, have to be removed. Flexible membrane processes are a promising alternative for small gas fields with varying nitrogen content. However, estimation for processing costs does not exist today. Existing membrane materials suffer from low to mediocre selectivity: also, rubbery and glassy membrane materials have inverse selectivities. This makes an optimal processes design crucial, but challenging. In this study an optimal membrane cascade is identified by mixed integer nonlinear programming. This rigorous and systematic method determines the most profitable process layout including membrane areas, feed and permeate pressure as well as recycle strategies and rates. We present optimized modules cascades based on (a) methane selective membranes only, (b) nitrogen selective membranes only and (c) combinations thereof using properties of industrially available gas separation membranes. Remarkably, the optimization identifies minimum costs for a process design using methane and nitrogen selective membranes combined. 40% process cost savings can be established if combinations of methane and nitrogen selective membranes are used. Furthermore we determined the optimal membrane properties on the upper bound. These potential membrane materials could even reduce process costs by 70%. However, process cost differs for the different membrane materials on the upper bound. In fact, this study showcases that our process optimization methodology can guide new polymer developments for nitrogen and methane selective materials. (C) 2015 Elsevier B.V. All rights reserved.