Methods for improving the sulfur-tolerance of nickel-based catalyst systems, as well as the improved catalyst systems, are disclosed. The methods can include adding praseodymium alone, or in combination with ruthenium and/or cerium, to a nickel-based catalyst system, thereby inhibiting sulfur poisoning of the catalyst system. Improved catalyst systems can have an added amount of praseodymium alone, or in combination with ruthenium and/or cerium, sufficient to inhibit poisoning of the system by sulfur.