We have performed pulsed-gradient NMR diffusion (D) measurements of five n-alkanes (24, 28, 36, 44, and 60 carbons) in a polyethylene (PE) host (molecular weight M = 33 kDa) as a function of concentration c (2-10 wt %) at 180 degrees C. Monte Carlo simulations on the second-nearest-neighbor diamond lattice (38, 46, 62, and 78 carbons) at c between 2% and 15% in a host of PE (M = 4.5 kDa) explored static and dynamic properties. The bridging method uses beads combining neighboring moieties and incorporates two-bead moves; it permits detailed reconstruction of the PE chain at any stage. It uses short-range rotational isomeric state and long-range intra- and interchain discretized Lennard-Jones potentials. For both experiment and simulation, trace D was obtained by extrapolating D(c) to c = 0 using the Fujita-Doolittle model with known chain-end free-volume parameters. A single ratio of 330 Monte Carlo steps per picosecond brings simulation into excellent congruence with experiment; this factor is identical to that required for PE melts. The applicability of the Rouse model is approached only for the largest alkanes, but the resulting alkane M dependence of trace D is seen to be in transition from the Rouse-like M-1 dependence to a steeper value characteristic of reptation with constraint release. Since the M dependence of D for alkane melts adheres to a nearly linear M-1.83 power law at this temperature, simulation and experiment confirm earlier theoretical considerations that the excess over M-1 arises entirely from the effects of the M-dependent host viscosity, or free volume, in the melts.