str.replace affects the if statement!

[Eslam]

I have two codes where using str.replace affects the performance of the if statement it lies in.
Can anyone tell me why this difference happens!!
Code#1

from random import randint
sen="I am a NOUN, and I'll always be NOUNing around, and all the NOUNs are gonna hate it"
i=0
while i<len(sen):
    if sen[i:i+4]=='NOUN':
        n=randint(0,3)
        if n==0:
            noun='tree'
        elif n==1:
            noun='horse'
        elif n==2:
            noun='bed'
        elif n==3:
            noun='sofa'
        print(noun)

        sen=sen[:i]+noun+sen[i+4:]
    i=i+1

print (sen)

wait=input('Enter a value ')

Code#2

from random import randint
sen="I am a NOUN, and I'll always be NOUNing around, and all the NOUNs are gonna hate it"
i=0
while i<len(sen):
    if sen[i:i+4]=='NOUN':
        n=randint(0,3)
        if n==0:
            noun='tree'
        elif n==1:
            noun='horse'
        elif n==2:
            noun='bed'
        elif n==3:
            noun='sofa'
        print(noun)

        sen=sen.replace(sen[i:i+4],noun)
    i=i+1

print (sen)

wait=input('Enter a value ')