Currently, there exist large heavy-oil reserves in countries like Venezuela and Canada. In Venezuela, heavy oil represents 69% of the reserves, and its exploitation is not always feasible using traditional pumping technologies. In particular, this is the case of some in-lake oil wells in Venezuela, which are impossible to exploit by means of any known efficient way of oil lifting. An alternative is the gas-chamber pumping (GCP), an intermittent artificial lift method used in diverse areas of USA, in shallow wells with heavy oil and in areas where a source of high-pressure gas exists. Few works are reported on the modeling of the phenomena associated to GCP the most rigorous being the one published by PDVSA-Intevep in the year 2000. This model, however omits some key aspects related with gas injection, which affects its precision to simulate or design GCP systems. The present work develops a model to rigorously simulate the stage of gas injection into the chamber incorporating aspects like the flow of gas from the supply manifold up to the wellhead, the gas expansion within the injection valve, the descending flow along a coiled tubing, and the heat transfer associated. The pressurization process and chamber venting are also modeled. The model predictions are in excellent agreement with experimental data