We investigate the reflectance of a dielectric plate coated with
 a graphene sheet which possesses the nonzero energy gap and chemical
 potential at any temperature. The general formalism for the
 reflectance using the polarization tensor is presented in the framework
 of Dirac model. It allows calculation of the reflectivity properties
 for any material plate coated with real graphene sheet on the basis of
 first principles of quantum electrodynamics. Numerical computations of
 the reflectance are performed for the graphene-coated SiO$_2$ plate at
 room, liquid-nitrogen, and liquid-helium temperatures. We demonstrate
 that there is a nontrivial interplay between the chemical potential,
 energy gap, frequency, and temperature in their joint action on the
 reflectance of a graphene-coated plate. Specifically, it is shown that
 at the fixed frequency of an incident light an increase of the chemical
 potential and the energy gap affect the reflectance in opposite
 directions by increasing and decreasing it, respectively. According to
 our results, the reflectance approaches unity at sufficiently low
 frequencies and drops to that of an uncoated plate at high frequencies
 for any values of the chemical potential and energy gap. The impact of
 temperature on the reflectance is found to be more pronounced for graphene
 coatings with smaller chemical potentials. The obtained results could be applied for
 optimization of optical detectors and other devices exploiting graphene coatings.