Compared　to　that　for　two-dimensional　(2D)　lattices,　our　understanding　of　heat　conduction　in　2D　gases　is　still　limited.　Here　we　study　heat　conduction　behavior　of　2D　gas　systems　with　momentum-conserving　and　-nonconserving　interparticle　interactions　by　using　the　nonequilibrium　and　equilibrium　molecular　dynamics　methods.　For　the　momentum-conserving　system,　we　find　that　when　the　dimensionality　of　the　system　is　changed　from　2D　to　quasi-one-dimensional　(quasi-1D),　the　heat　conductivity　kappa　diverges　with　the　system　size　L　as　kappa　similar　to　In　L　(the　theoretical　prediction　for　2D　systems)　for　a　short　L　and　shows,　in　the　thermodynamic　limit,　a　tendency　to　kappa　similar　to　L-1/3　like　that　predicted　in　1D　fluids.　This　suggests　that　the　dimensional-crossover　effect　of　heat　conduction　exists　in　2D　systems　with　conserved　momentum.　In　contrast,　for　the　momentum-nonconserving　system,　as　L　increases,　finite　heat　conductivity　independent　of　L　is　observed.　These　findings　are　in　agreement　with　the　predictions　given　by　hydrodynamic　theory　and　thus　further　confirm　the　validity　of　the　theory　in　2D　gases.