The key factors that affect the growth of methane hydrates are identified using molecular dynamics simulations. The three-phase molecular models consisting of methane gas, liquid water, and solid hydrate phase are used in this study. The melting temperatures of such a model at different pressures are found to be in good agreement with experiment. The growth rate of methane hydrate is found to be dominated by (1) the solubility of methane in the liquid phase, (2) the diffusivity of methane in water, and (3) the adsorption of methane by methane-filled incomplete water cages at the solid-liquid interface. The solubility, and hence the growth rate, increases with the partial pressure of methane in the vapor phase. The mass transport resistance from adsorption and the diffusion of methane are two competing factors, with the adsorption of methane at the interface found to be the rate-limiting step. The presence of a high concentration of incomplete clathrate hydrate cages presents strong affinity to dissolved methane at temperatures below the melting point. In addition to methane adsorption, water molecules must be expelled to form the complete clathrate cages. Both processes lead to a methane concentration minimum at 5-9 angstrom in front of the growing interface. The methane concentration minimum provides the driving force for methane transport from the bulk to the interface. There are two types of solid layers of methane hydrate in the (1,0,0) direction. The growths of these layers are different, highly correlated, and affected by the methane concentration. A detailed mechanism of the layer growth is deduced from our simulations.