We present a simple molecular dynamics (MD)-based method for determining the Henry's constant and gas-solubility in liquids and have applied it to the case of oxygen dissolved in liquid benzene. This method is an extension of an algorithm we presented earlier to study osmosis and reverse osmosis in liquid solutions and gaseous mixtures. It is based on separating a gaseous compartment in the MD system from the solvent using a semi-permeable membrane. This membrane is permeable only to the gas molecules. The Simulation system is then allowed to come to equilibrium at the desired density and temperature. After equilibration, the simulation is continued to determine the Henry's constant using a few simple thermodynamic relations. Since particle insertions or deletions are not needed in this method, it is free of any limitations in the high-density regime. We have compared our simulation results both with the experimental data and with the predictions from cubic equations of state. The simulation results show correct temperature dependence and also an excellent quantitative agreement with the experimental Henry's constant data. Predictions using equations of state are off by almost a factor of 2, in comparison with the experimental data, and show incorrect temperature dependence.