Owing to the potential of polymeric and nanocomposite membranes for industrial application in CO2 capturing and gas separation processes, permeation properties of CO2, N2 and O2 through the polymer matrix have been an object of extensive research. We measured the permeation rates of gases (pure and mixed gas) within a novel nanocomposite membrane composed of poly tetramethyleneglycol (PTMG), hexamethylene diisocyanate (HDI), and diamine chain extender, 4,4- methylenebis(2-chloroaniline) (MOCA) at various silica loadings and operating conditions. The novel polyurethane was prepared by a two-step bulk polymerization technique based on the molar ratios of the used constituents 1 : 3 : 2 for PTMG: HDI :MOCA, respectively. The FTIR spectra indicated that the extent of phase separation decreased by increase in the SiO2 content. From the DSC and XRD analyses, the existence of small crystalline areas within the soft and hard segments of matrix was proved. High thermal stability of new nanocomposites was authenticated by a 90 °C increase in the decomposition temperature upon including the SiO2 particles into the polymer matrix. By providing a longer diffusion path, a reduction in the permeation of penetrants occurred after the incorporation of SiO2 content. By raising the temperature from 25 to 45 °C, the gas permeation value of CO2, O2 and N2 rose steeply: 35, 54 and 81% in neat PU and 49, 64 and 137% in PU containing 15 wt%, respectively. Conversely, the obtained results for increasing the feed gas pressure from 6 to 10 bar revealed that the penetration of non-condensable gases, O2 and N2, decreased while the permeation rate of CO2 polar gas surged dramatically. Nevertheless, a simultaneous increment in the selectivity amounts of both gas pairs was revealed. For the gaseous mixtures, the trend of changes in permeability and selectivity values were almost identical with those of pure gas: decrease in permeation, and vice versa increase in gas pair selectivity. Eventually, the separation results of the prepared membranes indicated a strong tendency to move towards Robeson’s line by incorporation of SiO2 nanoparticles into the matrix of membranes.