Simple Caesar shifter in C#
Simpler Methods.

the code:


public string removeSpaces(string plainIn)
{
    plainIn = plainIn.Replace(" ", string.Empty);
    return plainIn;
}



Could be:

public string removeSpaces(string plainIn)
{
    return plainIn.Replace(" ", string.Empty);
}


Which begs the question... why have the removeSpaces method at all?

The same is true with toUCase()

Simpler Conversion

Why use Convert.ToInt32(....); and Convert.ToChar(...) when (int) and (char) will do?

Simpler Math

This line here (the crux of the problem), can be simplified using modulo and removing the Magic Numbers:


int Num = Convert.ToInt32(charArray[i]) + shift;
readOut += Convert.ToChar(Num &gt; 90 ? Num -= 26 : (Num &lt; 65 ? Num += 26: Num));



can be:

// Constants....
private const int LETTERA = (int)'A';
private const int LETTERCOUNT = (int)'Z' - LETTERA + 1;

// actual code.
 readOut += (char)(LETTERA + (((int)charArray[i] + shift - LETTERA + LETTERCOUNT) % LETTERCOUNT));


(See ideone here)

Without the conditional / ternary operator it is more likely to be tighter/faster code as well.

Comments

You have used magic numbers, so you attempted to document them with a comment:


public string shift(string plainIn, int shift)
{// 65=a 90=z in ASCII



But, the comments are wrong, and misleading (wrong case character).

 {// 65=A 90=Z in ASCII


Symmetry

When you have methods that do symmetrical work (they are closely related), then you should ensure the parameter names are the same.... For example, you have:


public string Encrypt(string ReadIn, int Shift)



and 


public string Decrypt(string ReadIn, int amountShift)



The second should have the parameter name Shift