Herein we present a rational design and fabrication of nitrogen-enriched hierarchical porous polymer networks by direct use of polyethylenimine (usually loaded on porous supports) as the building block. Gas adsorption is mainly dependent on the adsorption temperature and pressure. Therefore, adsorption of CO2, CH4, and N-2 was performed by varying adsorption temperature (273 and 298 K) and pressure (up to 1 bar). The fabricated polymer networks demonstrate the remarkable CO, uptake capacity and selective separation of CO2 over N-2 and CH4. Owing to specific acid-base interaction of nitrogen-enriched polymers with acidic CO2 and the molecular sieving effect of hierarchical pore structure in polymer networks, good CO2 uptake (199 mg-CI) and superior selectivity of CO2/N-2 (411) and CO2/CH4 (107) is observed at 273 K. The adsorption capacities and selectivities obtained in the present work are obviously higher than the reported functional benchmark adsorbents PCTF-1 (143.0 mg.g(-1)), PECONF-3 (145.2 mg.g(-1)) and UiO-66-NH2 (171.6 mg.g(-1)) as well as recently reported carbon-based and metal-organic framework adsorbents under analogous conditions. Moreover, perfect recyclability is observed without loss in 10 successive cycles with mild regeneration conditions. Thus, good CO2 uptake and superior CO2/N-2 and CO2/CH4 selectivities with perfect reusability for successive CO2 capture make hierarchical porous polymers a promising candidate for selective CO2 separation from various gas mixtures.