Polychlorinated-biphenyl (PCB) contamination problems are prevalent in the natural gas industry especially in the pipeline transmission and distribution sectors. As a result of PCB disposal problems, many natural gas companies have been subjected to heavy fines. Considering the magnitude of the problem, the gas industry has taken the initiative to pursue a comprehensive remediation effort to cleanup pipeline and distribution systems contaminated with PCB. This study is a part of the remediation effort. This paper addresses the phase behavior of PCB-contaminated natural gas systems and explores how to utilize this information to attempt the formulation of remediation strategies. A model suitably designed for studying the problem has been developed. The model is used to investigate the distributive nature of PCB compounds and to analyze their effects on the overall phase behavior of the natural gas system. New quality line definitions are introduced for the purpose of tracing PCBs in the gas mixture. Some experimental results and model predictions are presented and discussed. A case study using actual field natural gas mixture is performed in order to demonstrate the capability of the model prediction. The study is important in assisting engineers and operators to design an effective remediation strategy to cleanup pipeline transmission and distribution lines contaminated with PCBs.