I prefer the following translations:
“I don’t have the slightest doubt that she’s innocent.”
“I haven’t the slightest doubt that she’s innocent.”
The sentence in Portuguese doesn’t contain the verb “to believe,” nor the idiom “beyond the shadow of a doubt,” though I’m not denying that the given translation comes fairly cloze to the sentence’s meaning, but the first half appears incorrect.
The “beyond (without) a shadow of a doubt” expression exists in Portuguese. So your English sentence would properly be translated:
“Eu acredito, sem sombra de dúvida, que ela é inocente.”
Errei ou tenho razao?