Flame synthesis exhibits significant potential for mass-producing carbon nanotubes (CNTs). In this study, we have developed a computational fluid dynamics (CFD) model at the flame scale and employed a CNT growth rate model (GRM) to predict CNT growth within a quasi-pyrolysis chamber. The investigation explores the influence of fuel concentration in a methane diffusion flame on the resulting length of synthesized CNTs within this chamber. As methane concentration decreases via nitrogen dilution, shorter methane diffusion flame lengths are observed atop the burner, accompanied by a reduction in flame temperature. Notably, CNTs are not produced when the fuel concentration falls below 92 vol% due to the low temperature and minimal carbon concentration in the synthesis chamber. Interestingly, the maximum synthesized CNT length increases by 1.248 μm as the fuel concentration shifts from 100to 97 vol%. This increase is attributed to the conducive thermochemical conditions within the synthesis chamber. However, this trend reverses and culminates in zero length when the fuel concentration reaches 92 vol%. The temperature range of 875–905 K is identified as favorable for CNT growth, resulting in CNTs with lengths ranging from 4.0 μm to 4.7 μm.