  Heat transport in spin-boson systems near the thermal equilibrium is
systematically investigated. An asymptotically exact expression for the thermal
conductance in a low-temperature regime wherein transport is described via a
co-tunneling mechanism is derived. This formula predicts the power-law
temperature dependence of thermal conductance $\propto T^{2s+1}$ for a thermal
environment of spectral density with the exponent $s$. An accurate numerical
simulation is performed using the quantum Monte Carlo method, and these
predictions are confirmed for arbitrary thermal baths. Our numerical
calculation classifies the transport mechanism, and shows that the
noninteracting-blip approximation quantitatively describes thermal conductance
in the incoherent transport regime.
