In order to mitigate climate change, the countries that ratified the Kyoto protocol agreed to lower their greenhouse gas emissions during the period 2008-2012 of 5% on average compared to their emissions of 1990. Carbon separation and storage (CSS) processes contribute to this effort. In this paper, a carbon separation process based on polyphenylene-oxide membranes is considered. The objective is to estimate (through numerical simulations) the costs of separating the carbon dioxide produced by a natural gas cycle power plant with this technology and to compare it with values for other technologies found in literature. After the specifcation of the source (a gas turbine), the behaviour of a single membrane is simulated. The program used for the simulations is coded in Matlab and AMPL. Then a model consisting of two membranes in series is presented and followed by the corresponding results. The cost functions of the membrane module (membranes and compressors) are introduced. This allows the energy cost penalty and the plant efficiency penalty related to the carbon separation process to be calculated. Finally, the price of the ton of separated CO2 is computed and compared to values found in articles.