A novel method to predict gas solubility in cis-1,4-polyisoprene is developed using molecular dynamics (MD) simulations under constant particle number, constant pressure and constant temperature (NPT) conditions. Analogous to the experimental sorption technique, the binary-phase model constructed of gas/polymer was prepared. In order to maintain external pressure of the whole system during long NPT-MD runs, the vapor phase was filled with virtual liquid which has no interaction with the gas molecules and has only a repulsive interaction with the polymer. After attaining equilibration of the system, the solubility of oxygen and carbon dioxide in the polymer phase were estimated in the temperature range from 273 K to 373 K by counting the number of gas molecules inside the polymer phase. The average solubility linearly increased with the increase in the external pressure, indicating that Henry's Law was satisfied. The solubility coefficient obtained from the present method showed good agreement with the experimental data. Concentration profiles of gas molecule showed that it was significantly higher near the interface than both in the gas phase and inside the polymer phase. The distinction between "adsorbed" and "absorbed" gas molecules and also their effect on the solubility was discussed.