Answer:
 Incomplete combustion 
Step-by-step explanation:
CH4 + 2O2 → CO2 + 2H2O Incomplete combustion – carbon monoxide and water. 
combustion reactions 
Exothermic Reaction In this type of reaction, generally energy in form of heat is released. CH4 + 2O2---- CO2 + 2H2O + energy Methane Oxygen Carbon dioxide For example all combustion reactions are exothermic.