The effect of spectral radiative properties of fly-ash particles on radiative heat transfer through an inhomogeneous planar layer near the walls of a combustion system is investigated. The spectral absorption and scattering coefficients, as well as the scattering phase function of the particles, are obtained from the Lorenz-Mie theory as functions of temperature and particle diameter. These data are employed in the solution of the radiative transfer equation, which is modeled using a double spherical harmonics approximation. In the calculations, a linearly anisotropic scattering phase function is used. It is shown that, if the medium temperature distribution is nonuniform, the divergence of the radiative flux term calculated using the spectral fly-ash properties differs significantly from that obtained using constant properties. This means that if fly-ash properties are assumed wavelength independent, a coupled radiation-convection heat transfer analysis would underestimate the gas temperature near the walls of a typical furnace. On the other hand, the temperature dependency of the fly-ash radiative properties is quite weak and can safely be neglected.