Liquid condensation in natural gas transmission pipelines commonly occurs due to the thermodynamic and hydrodynamic imperatives. Condensation subjects the gas pipeline to two-phase transport. Neither the point along the pipeline at which the condensate is formed nor the quantity formed is known a priori. Hence, compositional multiphase hydrodynamic modeling, which couples the multiphase hydrodynamic I model with the natural gas phase behavior model, is necessary to predict fluid dynamic behavior in gas/condensate pipelines. A transient compositional multiphase hydrodynamic model for transient gas-condensate two-phase flow in pipelines is presented. This model consists of our newly developed well-posed modified Soo＇s partial pressure model in conservative form which serves as the transient multiphase hydrodynamic model, and the phase behavior model for natural gas mixtures.