Injection of nitrogen mixed with produced hydrocarbon gas is underway in a miscible displacement project in the Chunchula field, a Smackover dolomite rock containing undersaturated oil and gas phases in the same reservoir. An equation of state compositional simulator was used to describe the fluid and phase variations in the reservoir, match history, and to forecast field performance under the enhanced recovery operation. The numerical model was notably useful in optimizing oil recovery from regions of contrasting permeability. The model study showed that the efficient revaporization recovery mechanism can be interrupted and left incomplete as a result of shutting in producers, even though injection continues. A continuing history match of well pressure and gas-oil ratio data with the model will further improve our understanding of the heterogeneous reservoir rock.