Carbon dioxide (CO2) capture and separation are two currently accepted strategies to mitigate increasing CO2 emissions into the atmosphere due to the burning of fossil fuels. Here, we show the simulation results of hydrate-based CO2 capture and selective separation from the CO2/H2 mixture dissolved in water, both using single-walled carbon nanotubes (SW-CNTs). The spontaneous formation of quasi-one-dimensional (Q1D) polygonal CO2 hydrates under ambient pressure was observed within SW-CNTs immersed in CO2 aqueous solution. Moreover, highly selective adsorption of a CO2 over a H2 molecule is observed in the Q1D polygonal ice nanotube due to a much lower value of the potential mean force (PMF) difference for a CO2 molecule than for a H2 molecule enclosed in the corresponding hydrate. The simulation results indicate that the formation of Q1D hydrates can be an effective approach for CO2 capture or for the separation of CO2 from H2 in the mixture.