This work presents a kinetic Monte Carlo model to simulate noble gas retention in amorphous solid H2₂O–CO2₂ ice mixtures under varying thermal conditions. Calibrated with experimental temperature-programmed desorption data and temperature–density relations, the model enables long-term simulations in small (~150 nm) ice grains. It shows efficient noble gas retention at ≤30 K, with significant loss near 40 K. Krypton fractionation occurs mainly in ices formed at these warmer temperatures. Using protosolar gas abundances, the model reproduces the noble gas composition measured in comet 67P/Churyumov–Gerasimenko. Results suggest the comet’s bulk formed near 40 K, while its icy grains may trace back to colder (~10 K) presolar reservoirs, preserving signatures of both local and interstellar environments.