Non-condensable gases (NCG) are co-injected with steam in SAGD to reduce the steam oil ratio (SOR) without negative effects on the oil production rates. However, most simulation studies show that NCG accumulates at the boundaries of the steam chamber, reducing the oil production rates. In this work, a simulation study was conducted to investigate the parameters affecting the response of NCG co-injection in SAGD. It is found that the gas-liquid relative permeabilities are the parameter that most affects the simulation results. Three experiments were conducted to extract equivalent thermal gas-liquid relative permeabilities curves. With the experimental curves, the simulation shows that small amounts of NCG reduce the SOR, without a negative impact in the oil recovery. Also, it is shown that most of the injected gas leaks from the steam chamber, and it is not necessary to wait until late times of the project to start the NCG co-injection.