We are interested in the design of hybrid organic-inorganic adsorbents (HOIAs) for gas separation and storage. In this paper, we propose a methodology for the design of HOIAs based on periodic mesoporous silicas. We use a method based on kinetic Monte Carlo simulation to generate realistic model adsorbents and then carry out Grand Canonical Monte Carlo simulations of adsorption in these model materials. Good agreement is obtained between the adsorption behavior of the model adsorbents and that of the corresponding real materials, giving confidence that we can identify optimal structures for particular gas-separation applications. We demonstrate this capability using, as an example, the pressure-swing adsorption of carbon dioxide from flue gas, using a HOIA based on MCM-41.