The objective of this work was to develop a kinetic analysis method by using a dynamic model that accounts for the thermal decomposition behavior of polymers with the variation of the conversion. The proposed method was applied to predict the thermal decomposition of polyethylene. The kinetic analysis was studied by conventional thermogravimetric technique with various heating rates in nitrogen atmosphere. To verify the appropriateness of the proposed method, the results from this work were compared with those of various analytical methods and the literature. The TG data were also compared with the values calculated by using the kinetic parameters from the dynamic method. It was found that the dynamic method gave a reliable value of kinetic parameters, and the activation energy and the reaction order of thermal decomposition of high-density polyethylene were larger than those of low-density and linear low-density polyethylene.