Is replace a noun?
The word 'replace' is a verb: replace, replaces, replacing, replaced.
The noun forms for the verb to replace are replacement and the gerund, replacing.
The pronoun that would replace the possessive noun Sheila's is her (her picture). The pronoun 'her' is a possessive adjective, a word used to describe a noun. The pronoun that would replace the noun phrase Sheila's picture is it. The pronoun 'it' is a personal pronoun, a word that takes the place of a noun for a specific thing.
Antecedents can be any noun (or noun form) where pronouns will replace the repetition of the noun. The most common pronouns that replace antecedents are personal pronouns (I, me, he, she, it, we they) or possessive adjectives (my, your, his, her, its) or possessive pronouns (his, hers, theirs, mine, yours).
The word 'replaces' is not a noun. The word 'replaces' is the third person, singular, present of the verb to replace. The noun forms of the verb to replace are replacement and the gerund, replacing. A noun is used as the subject of a sentence or a clause, and as the object of a verb or a preposition. Examples: I've ordered a replacement for the old air conditioner. (direct object) Replacing you will be difficult…
The CAT is chasing a mouse. This noun is the subject of this sentence. Some other examples (replace the highlighted word 'noun' with a noun of your choosing): Noun of a feather flock together. A noun and his money are soon parted. A noun in the hand is worth two in the bush. A noun saved is a noun earned. A noun a day keeps the doctor away. All noun and no noun make Jack…