Lennard-Jones chains grafted to solid surfaces in a supercritical solvent are simulated with a continuum grand canonical Monte Carlo method. The force of interaction between two surfaces is calculated as a function of solvent density and temperature and analyzed as a function of the conformational properties of the grafted chains. At high, liquidlike bulk solvent densities, the chains are solvated and the interaction forces are repulsive. As the solvent density is lowered, the chains collapse, and the surfaces become attractive, indicating flocculation. The critical flocculation density coincides with the critical solution density for a bulk mixture of chains and solvent (corrected for local density enhancement). The bulk critical solution density, in turn, corresponds to the coil-to-globule transition of a single chain in bulk solution. The predicted correspondence between these properties agrees with results from lattice-fluid self-consistent field theory and colloid stability experiments. In good and poor solvents, the range of the interaction force between surfaces is much longer than the length of the grafted chains, due to expulsion of solvent from the interface as the surfaces are compressed. Very similar ranges were seen for forces measured with the surface forces apparatus in liquid solvent [G. Hadziioannou et al., J. Am. Chem. Sec. 108, 2869 (1986)].