The effect of hydrocarbon gas injection on oil production during Steam Assisted Gravity Drainage (SAGD) projects was investigated using numerical simulation. The results indicate that oil production rates as well as total oil production are significantly reduced when gas is injected with steam from the early period of a SAGD operation. This is because most of the injected gas gathers at the upper part of the leading edge of the steam chamber and prevents the growth of the steam chamber there, which results in a reduction of ultimate oil recovery. However, if the gas injection is initiated during later periods of the process, an improved steam oil ratio is obtained without significant reduction in oil production rates and the total oil production. In this case, the injected non-condensable gas migrates to the upper part of the reservoir and does not prevent the growth of the steam chamber, since the chamber had already grown to the desired size. Gas injection slows down the growth of the steam chamber in the upper part of the reservoir and induces its growth downwards. This mode of growth of the steam chamber results in an improvement in steam oil ratio as is illustrated in this paper. An understanding of this mechanism and its optimal timing are important in the enhancement of the SAGD process.