Presented is a reducedorder thermal fluid dynamic model for gas/liquid twophase flow in pipelines. Specifically, a twophaseflow thermal model is coupled with a twophaseflow hydraulics model to estimate the gas and liquid properties at each pressure and temperature condition. The proposed thermal model estimates the heattransfer coefficient for different flow patterns observed in twophase flow. For distributed flows, where the two phases are wellmixed, a weightbased averaging is used to estimate the equivalent fluid thermal properties and the overall heattransfer coefficient. Conversely, for segregated flows, where the two phases are separated by a distinct interface, the overall heattransfer coefficient is dependent on the liquid holdup and pressure drop estimated by the fluid model. Intermittent flows are considered as a combination of distributed and segregated flow. The integrated model is developed by dividing the pipeline into segments. Equivalent fluid properties are identified for each segment to schedule the coefficients of a modal approximation of the transient singlephaseflow pipelinedistributedparameter model to obtain dynamic pressure and flow rate, which are used to estimate the transient temperature response. The resulting model enables a computationally efficient estimation of the pipelinemixture pressure, temperature, twophaseflow pattern, and liquid holdup. Such a model has utility for flowassurance studies and realtime flowcondition monitoring. A sensitivity analysis is presented to estimate the effect of model parameters on the pipelinemixture dynamic response. The model predictions of mixture pressure and temperature are compared with an experimental data set and OLGA (2014) simulations to assess model accuracy.