This study was conducted to elucidate relationships among various phenolic fractions in, and methane (CH4) emissions from, tropical plants when incubated in ruminal fluid in vitro. As a second objective, principal component analysis (PCA) was tested for its utility in screening plants for their ability to reduce CH4 formation at simultaneously acceptable nutritional quality. Leaves from 27 tropical plants were analyzed for their nutritional composition and various phenolic fractions. They were incubated in vitro using the Hohenheim gas test method. Variables measured after 24 h of incubation were total gas and CH4 production, and pH, ammonia, bacterial and protozoal counts, as well as short-chain fatty acids in the incubation fluid. In vitro organic matter (OM) digestibility was computed by a standard equation. The data obtained was subjected to analysis of variance, correlation, regression and PCA. Among phenolic fractions, total phenols had the closest relationship with CH4/digestible OM (r = −0.84, P<0.001). The total tannin fraction contributed strongly to this effect (r = −0.74, P<0.001) whereas the non-tannin phenol fraction was less important (r = −0.45, P<0.05). Methane reduction by the influence of non-tannin phenols was not associated with a negative effect on protein degradation, while this was the case with tannins. Condensed (r = −0.60, P<0.01) and hydrolysable tannins (r = −0.60, P<0.01) contributed to the decrease in CH4/digestible OM. The loading plot of PCA showed that dietary crude protein (CP) content and incubation fluid ammonia, total short-chain fatty acids, propionate, valerate, iso-butyrate, iso-valerate as well as in vitro OM digestibility were clustered. They had inverse directions to contents of fiber fractions and incubation fluid acetate proportion and acetate-to-propionate ratio. The methane-to-total-gas ratio had the opposite effect of the contents of any phenolic fraction. Plants possessing a favorable forage quality, based on the corresponding PCA score plot, were Carica papayaManihot esculentaMorinda citrifoliaSesbania grandiflora and Melia azedarach, whereas CH4 mitigating plants included Swietenia mahagoniAcacia villosaEugenia aqueaMyristica fragrans and Clidemia hirta. All phenolic fractions studied reduced CH4 emissions from in vitro incubations with ruminal fluid and PCA seems useful to screen plants for high nutritional quality and low ruminal CH4 formation. However, high forage quality seemed to be partially associated with high CH4 emission. The search for plants rich in non-tannin phenols might be promising as these compounds appear to decrease CH4 while they obviously have less negative effect on protein degradation as compared to the tannin fractions.