In North America, most domestic waste produced is disposed in landfills. These sites generate leachate and gas, mainly CH4 and CO2, which are harmful for the environment if not properly controlled. The design of active landfill gas recovery systems is based in large part on the radius of influence of vertical pumping wells. This parameter is commonly estimated empirically. This study presents results of numerical simulations of the radius of influence of gas recovery wells for different site conditions. The simulations were performed with the TOUGH2-LGM simulator. In the simulation scenarios, the radius of influence was defined in relation to several factors: the waste thickness, the generation rate of CH4 gas in the waste, and CH4 concentration in the recovered landfill gas. The results are presented in the form of general graphs that are not site-specific. The adequacy of the results still needs to be validated against field measurements. On the basis of simulation results, a systematic approach is proposed for the design of landfill gas recovery systems, and this approach is illustrated with a hypothetic example. This approach should guide landfill managers and engineers in the design of landfill gas recovery systems. The simulations only considered cases where landfills are open to the atmosphere, which are representative of most operating conditions. The results thus do not apply to post-closure conditions usually involving an impermeable cover built on top of the waste.