Gas wells frequently exhibit changing storage during a transient test because of high fluid compressibility. Further complications may arise owing to heat exchange between the wellbore fluid and the formation, especially in high-temperature reservoirs. Thus, fluid temperature changes during a transient test, thereby complicating test interpretation when surface measurements must be used in a hostile downhole environment. This paper describes the application of a wellbore/reservoir simulator. This simulator can be run in two modes. In forward modeling, the wellhead pressure, temperature, and bottomhole pressure can be computed as a function of time, given reservoir parameters and well-completion details. In contrast, reverse simulation directly translates measured wellhead pressure (WHP) and temperature (WHT) to bottomhole pressure (BHP) for subsequent analysis. Forward simulation of wellbore pressure and temperature is a valuable tool for designing transient well tests. In particular, in a hostile downhole environment, the ability to estimate BHP accurately from wellhead measurements is an invaluable asset where downhole measurements may not be cost effective. Even in favorable situations, the simulator can aid both rest design and interpretation. Application of the simulator to field examples has helped us gain considerable insight into the mechanics of transient flow in the wellbore. For example, we observe a denser fluid toward the wellhead rather than at the bottom of the well because of the temperature effect. Consequently, decreasing storage behavior is observed at the bottom of the well. Two field examples obtained from a Gulf Coast gas field art used to illustrate the capabilities of the simulators. Good agreement is noted between the computed and measured BHP's during both types of simulations.