Cellulose acetate, some polyimides and polysulfones are mainly used in commercial membranes for gas separation processes. Although these polymers meet the desired combination of properties, the design of novel polymers possessing improved permeability and/or separation selectivity of gases could lead to enhanced gas separation efficiency and minimized energy costs. Here, we report gas permeability data of two isomeric vinyl-addition polymers derived from available ester-functionalized norbornenes that exhibit remarkable ideal and mixed gas selectivity as well as permeability for industrially important pairs of gases (CO2 with N2 and CH4). In particular, polynorbornene bearing acetoxy side groups (AcPNB) demonstrated carbon dioxide permeability of 270 Barrer and selectivity of CO2/N2 separation above 20, superposing to that of cellulose acetate (CA) in CO2 separation performance. The polymer containing methoxycarbonyl groups (McPNB), which is an isomer of AcPNB, showed even higher separation performance for CO2-containing gas mixtures. Carbon dioxide permeability and selectivity of CO2/N2 for McPNB exceeded 350 Barrer and 50, respectively. The data on CO2/N2 separation for McPNB are on the upper bound of Robeson plot of 2008 year. Mixed gas experiments performed for binary mixtures of CO2 with N2 and CH4 confirmed promising gas-separation parameters of the studied polymer, displaying attractive CO2/gas selectivity and CO2 permeability. Therefore, considering the simple and cheap syntheses of these polymers and the CO2 separation performance, they can be considered as promising candidates as selective layers for membrane gas separation processes.