The frequency, chemical potential, hopping energy and temperature dependence of polarization in a graphene-topological-insulator heterostructure is investigated. The polarization is found to be sensitive to the graphene-topological-insulator hopping energy and chemical potential. Compared to hopping energy and chemical potential, temperature has a relatively small effect on the polarization which only slightly changes the peak value. The unique band structures of graphene-topologicalinsulator heterostructures give rise to dual polarization peaks. Furthermore, the position of the polarization peak that originates from the graphene bands is robust while the position of the other peak can be tuned by varying the hopping energy and chemical potential. From the polarization function two branches of plasma dispersion are observed due to the coupling of graphene and the surface states of topological insulators.