Coinjecting solvent with steam under a steam-assisted-gravity-drainage (SAGD) process to reduce the required steam for production has gained importance in recent years. An extensive 2D simulation study to better understand the drainage mechanism of steam-with-solvent coinjection in the SAGD process shows that the condensation time difference of solvent and steam results in different films of gas solvent, liquid solvent, and water along the fluid interface. There is an optimal solvent-type and -concentration-ratio range for a particular reservoir and operating condition. Coinjecting the solvent at low concentration ratios can take advantage of the solvent dilution effect without losing too much heat effect from steam. In addition, this study indicates coinjection of suitable solvent mixtures may lead to better performance than injection of pure solvent in the field.