A life-cycle assessment (LCA) study on various recycle routes of plastic materials has been conducted using the case of polyethylene terephthalate (PET) bottles as an example. The energy consumed and the emissions released during the entire life-cycle of the plastic material were accounted for using the energy and material balances on each stage of the life-cycle. A mathematical model including a simple nonlinear relation for the collection process of the bottles was derived for the system which encompasses all possible recycle alternatives. This model contains several adfustable parameters representing each alternative step of the recycle routes. Then through parameter sensitivity analysis ad optimization analysis we could both identify environmentally favorable recycle routes and determine the optimal conditions for the best one. The methodology of this study can be easily applied to the comparison of the general waste management alternatives determining their relative advantages and disadvantages viewed from the associated environmental burdens. Those results will be reported elsewhere. Life-Cycle Assessment (LCA), Nonlinear Programming (NLP), Multi-Objective Optimization, PET Bottles, Plastics Recycling Routes, Sensitivity Analysis