As SAGD moves from pilot test to commercial operation, a number of issues need to be dealt with. These include diagnosing and solving operational problems and improving energy efficiency. One of the methods of improving energy efficiency is to prolong oil production after steam injection stops by using the energy remaining in place. The results of a laboratory experiment and corresponding numerical history matching are reported in this paper. The study showed that the hot chamber continued its expansion after steam injection was stopped and a gas injection was initiated. The continuous expanding period represented the most productive period in the gas injection wind-down process. A total of 12.5% of OOIP was recovered during wind-down. Successful history matching of both the oil production curve and temperature profiles at different times demonstrated that the numerical simulation could handle the gas/steam mixing phenomena. Gas concentration profiles from numerical simulation indicated that gas was concentrated at the region where oil saturation was experiencing big changes.