...
for ch in message:
    code_val  = ord(ch) + key
    if code_val > ord('z'):
        code_val -= 26  # better: ord('z') - ord('a'), just in case len(alphabet) != 26
    codedMessage = codedMessage+chr(code_val)
...

...
for char in message:
    # check if the character is alphabetic
    if char.isalpha():
        ...

message = "just a simple test!"
key = 11
coded_message = ""
for ch in message:
    code_val  = ord(ch) + key
    if ch.isalpha():
        if code_val > ord('z'):
            code_val -= ord('z') - ord('a')
        coded_message = coded_message + chr(code_val)
    else:
        coded_message = coded_message + ch
print message
print coded_message

just a simple test!
ugef l etxbwp fpef!

def main():
 
    # gets user input to encode or decode
    decision = raw_input("Enter 1 to encode a message, and 2 to decode a message. ")
 
    if int(decision) == 1:      # starts encoding process if user enters 1
 
        # creates files for reading and writing to
        infile = open("plaintext.txt", "r")
        outfile = open("ciphertext.txt", "w")
 
        data = infile.read()
        key = int(data[0])      # "picks off" the shift key value from the text file and makes into an integer
 
        message = data[1:]      # slices off the shift key number to leave only the message to be encoded
        encrypted_message = ""
 
        # creates a loop that goes through each character in the string
        # and adds the shift key value to that character's ascii value,
        # then turns each of those ascii values back into a character
        for i in message:
            letter_value = ord(i) + key
            if letter_value > ord("z"):
                letter_value -= ord("z") - ord("a") + 1
            encrypted_message = encrypted_message + chr(letter_value)
 
        outfile.write(str(key))   # makes the key value a string and then writes to file
        outfile.write(encrypted_message)    # writes encrypted message to file
 
        infile.close()      # closes infile
        outfile.close()     # closes outfile

s = "abcdefg\n"
 
print s
print '-'*10
 
# strip off trailing '\n' char
s = s.rstrip('\n')
 
print s
print '-'*10