First and second combustors are provided, and each combustor includes a fuel nozzle and a combustion chamber downstream from the fuel nozzle. Each fuel nozzle includes an axially extending center body, a shroud that circumferentially surrounds at least a portion of the center body, and vanes that extend radially between the center body and the shroud. A first fuel port through at least one of the vanes is located at a first axial distance from the combustion chamber, a second fuel port through the center body is located at a second axial distance from the combustion chamber, and the vanes are located at a third axial distance from the combustion chamber. The system varies one or more of the first, second, and third axial distances from combustor-to-combustor to produce a combustion instability frequency in the first combustor that is different from the combustion instability frequency in the second combustor.