An extension to a high-order model for the direct isotropic/smectic-A liquid-crystalline phase transition was derived to take into account thermal effects including anisotropic thermal diffusion and latent heat of phase ordering. Multiscale multitransport simulations of the nonisothermal model were compared to isothermal simulation, showing that the presented model extension corrects the standard Landau-de Gennes prediction from constant growth to diffusion-limited growth under shallow quench/undercooling conditions. Nonisothermal simulations, where metastable nematic preordering precedes smectic-A growth, were also conducted, and novel nonmonotonic phase-transformation kinetics were observed.