Configurational-bias Monte Carlo simulations in the Gibbs ensemble (CBMC-GE) are used to investigate the adsorption of both linear and branched alkanes (ethane, propane, n-butane, and 2-methylpropane) from dilute solutions in liquid methane onto a carbon slit pore at T = 160 K and at either the saturation pressure or p_ext = 100 atm. Thermodynamic properties (adsorption isotherms, selectivities, and Henry's law constants) and structural properties (density and orientational distributions) are presented. Both the Henry's law constants and the separation factors depend exponentially on the number of carbon atoms for the linear alkanes, whereas chain branching and higher pressure lead to a reduction of these properties. The solute density profiles show oscillatory behavior along the surface normal, and peaks in the number density are correlated with a preference for parallel orientations. The CBMC-GE approach allows for the efficient calculation of these selective adsorption phenomena, and data for multiple solutes (in the dilute regime) can be extracted from a single simulation.