Cryogenic separation is one of many CO2 capture technologies developed for upgrading biogas or for CO2 separation from flue gases. In order to better understand the CO2 frosting phenomena, in this paper, CO2 frost growth is modelled considering deposition on a flat plate. The frost layer is modelled as a porous medium with heat and mass transfer occurring on the surface and within the frost layer. Simulations are conducted for CO2 separation from CH4+CO2 (biogas) and N2+CO2 (flue gas) mixtures, frost thickness and density were assessed. The modelling results are compared to experimental data found in literature and it is for the first time possible to have detailed and accurate model of heat and mass transfer for CO2 frosting. Multiple heat transfer correlations were assessed and improvement of accuracy between calculated and experimental results has reached 80 %. For CH4+CO2 mixtures, a sensitivity study is carried out. The drivers of frost formation are mainly plate temperature and inlet CO2 concentration, and to lesser extent the gas temperature.