A modified model is developed to determine initial gas in place and to predict the performance of any gas reservoir by using material balance approach and linear programming technique. Compaction of the aquifer rock, connate water expansion and/or expansion of water in the aquifer, shale water influx, and water enroachment from finite bounded reservoirs are included in the developed mathematical model. Each of these mechanisms are examined and their effects on the results of initial gas reserves and reservoir behavior are presented.