In theory, the combination of inorganic materials and polymers may provide a synergistic performance for mixed-matrix membranes (MMMs); however, the filler dispersion into the MMMs is a crucial technical parameter for obtaining compelling MMMs. The effect of the filler distribution on the gas separation performance of the MMMs based on Matrimid (R)-PEG 200 and ZIF-8 nanoparticles is demonstrated. The MMMs were prepared by two different membrane preparation procedures, namely, the traditional method and non-dried metal-organic framework (MOF) method. In CO2/CH4 binary mixtures, the MMMs were tested under fixed conditions and characterized by various methods. Finally, regardless of the MMM preparation procedure, the incorporation of 30 wt % ZIF-8 nanoparticles allowed to increase the CO2 permeability in MMMs. The ZIF-8 dispersion influenced significantly the separation factor.