We present accurate path integral calculations of quantum rate constants for model nonadiabatic reactions in condensed matter. The model is described by two coupled diabatic potential surfaces interacting Linearly with a bath of harmonic oscillators. The rate constant is obtained from the time integral of the flux-flux correlation function which is evaluated by the quasi-adiabatic propagator path integral method. We study the dependence of the reaction rate on friction, temperature, and exothermicity and compare with predictions of analytical theories. In particular, we observe a broad golden rule plateau as well as rate enhancement due to quantum resonances for low friction in agreement with the semiclassical analysis of Onuchic and Wolynes.