Your premise is incorrect. Substitution and hydrogenation do not both involve the removal of hydrogen atoms from alkanes.
Substitution in an alkane involves the removal of a hydrogen atom and its replacement with something else.
An example is the chlorination of methane.
H₃C-H + Cl-Cl → H₃C-Cl + H-Cl
Alkanes do not undergo hydrogenation. Hydrogenation is the addition of hydrogen to an alkene.
Hydrogenation does not remove H atoms from the alkene. Instead, it adds H atoms to the alkene.
An example is the hydrogenation of ethene.
H₂C=CH₂ + H-H → H-CH₂-CH₂-H