The mass accommodation (condensation) coefficient alpha of water vapor into liquid water was theoretically studied via two complementary approaches: by molecular dynamics (MD) scattering simulation and by computational fluid dynamics simulation of the droplet train/flow reactor experiment. The MD scattering simulation predicts alpha similar to 1 at 273 K. The fluid dynamics simulation quantitatively interprets the gaseous resistance in the droplet train flow tube, which demonstrated that the results of the droplet train/flow reactor experiment [Li et al., J. Phys. Chem. A 2001, 105, 10627] are consistent with a value for the water alpha in the range between 0.2 and 1. Both methods are thus seen to be consistent and provide values for the mass accommodation coefficient of water in the ideal situation free from surface impurities and nonequilibrium latent heat production.