This study investigates, by means of numerical modelling, the impacts of naturally occurring, as well as continuously coinjected and intermittently coinjected noncondensable gas (NCG), at different stages of the steam-assisted gravity-drainage (SAGD) process. The CMG Builder software was used to construct a homogeneous 2D baseline model based on generic Athabasca-type reservoir properties and well configuration. A fluid-component model was generated using the CMG WinProp package for modelling the phase behaviour and properties of reservoir fluids. This fluid-component model was modified to incorporate manually calculated K-values for the gas-water and the gas-bitumen phase equilibria. The modified fluid model was then incorporated into the baseline model and the CMG STARS thermal reservoir simulator. The simulation results of this study show that methane coinjection along with steam is generally not beneficial. Although it can reduce the heat loss to the overburden to some extent, the reduction in oil-drainage rate and total oil recovery negates the benefits of such heat-loss reduction. The poor performance of NCG addition to SAGD results from the tendency for the NCG to remain in the vicinity of the steam chamber and reduce heat transfer into the cold bitumen at the edges of the steam chamber, thereby impeding steam-chamber growth. Accurate modelling of NCG addition to steam in SAGD is highly dependent on the availability of appropriate gas-solubility data. NCG with steam may perform better compared with steam-only injection in SAGD if methane coinjection were investigated using a heterogeneous model in which SAGD is affected adversely by the presence of reservoir heterogeneities in the form of shale barriers, inclined heterolithic strata (IHS), and steam-thief zones.