The evolution of an SNR in a high-density medium has been studied using analytical and numerical hydrodynamic and photoionization models. The models show that SNR properties (luminosity, density, size, kinematics, emission line ratios, etc.) are essentially identical to those considered typical of the broad-line region (BLR) of radio-quiet active galactic nuclei (AGN). A model is presented in which the only free parameter is the density of the circumstellar medium. This model combines analytical and numerical hydrodynamic simulations with static photoionization computations to reproduce most of the basic properties of the BLR in a low-luminosity AGN. The computed line intensity ratios are found to be in excellent agreement with those observed in the BLR, and the model output shows the following features: bolometric luminosity of more than 10 exp 43 erg/s; emission line width of about 5000 km/s; size of about 0.01 pc; and total mass of ionized gas in the BLR from about 1 to 10 solar masses.