Three conventional and abundant municipal wastes (PET bottles, bread, and human hair) were used for the synthesis of the solid carbon product. The activation of this carbon powder was performed with the aid of KOH and ZnCl2 chemicals to achieve activated carbons (ACs). The characterization methods, such as SEM, FTIR, N2 adsorption-desorption analysis, BET, and DFT pore size analysis, were applied to characterize the synthesized mixed matrix membranes (MMMs). The ACs synthesized by PET wastes and human hairs showed the highest and lowest sorption capacity, BET surface area, and pore volume, respectively. The activation of ACs using KOH showed overall better effectiveness in CO2/CH4 separation performance of fabricated MMMs compared with using ZnCL2, due to the presence of hydroxyl groups at the surface of KOH-modified ACs. The developed model for the spiral wound membrane module showed good agreement with experimental data and modeling results of the hollow fiber module in the literature. The result of the model on the best-performed membrane showed that the increase in module length and diameter led to a logarithmic increase in the stage cut. It appears that the increase in module diameter is more practical and beneficial than that in module length. The result of the simulation of a double step with recycling of permeate (DoSRP) separation system in the Aspen Plus environment shows that the increase in CH4 content of the feed, temperature, and decrease in thickness of membranes all have deteriorative effect on the separation performance of overall DoSRP configuration separation system. It was also deduced that MMMs with higher CO2 permeance and CO2/CH4 ideal selectivity suffer more from the mentioned changes in simulated manipulated separation parameters compared with less effective MMMs.