A mathematical model is developed to simulate a blowdown of a long pipeline conveying a flashing multicomponent mixture. The effect of the thermal capacitance of the pipe was incorporated into the model by employing a novel approach lit the formulation of the energy equation for the fluid flow in the pipeline. The model was validated by comparison with previously published experimental data for the blowdown of a pipe containing a two-component mixture. A detailed study was performed to investigate the effect of heat transfer between the fluid flow and the pipe wall on the process of depressurization of a long pipeline. Numerical simulations were carried out to assess the effect of friction on the blowdown rime. The results obtained lead to a better understanding of the blowdown phenomenon and can be used for predicting the consequences of controlled or uncontrolled releases of flashing liquids from long pipelines.